About 35% of Americans experience balance problems such as vertigo, lack of coordinated movements, and dizziness. Balance problems can severely reduce quality of life, add to high healthcare costs, and lead to premature death due to falls. A major cause of balance disorders is the loss of vestibular hair cells in the inner ear, which convert head movements into electrical signals that are sent to the brain via the vestibular nerve. Hair cell death has many causes, including gene mutation, injury, infection, therapeutic surgery and drug treatments, and aging. The proposed project brings together the expertise of two investigators who share the following goals: to understand the fundamental mechanisms underlying vestibular hair cell function and to develop ways to treat balance disorders resulting from hair cell injury or loss. The Stone lab at the University of Washington (UW) recently discovered that one type of vestibular hair cell - type II - in adult rodents has a feature that has never been described: one or more extensions of cytoplasm (or processes) that project laterally from the base of the cell, sometimes over several cell lengths. These processes have a variety of shapes and seem to contact other cells in the epithelium, including other type II hair cells. This latter observation raises the novel idea that direct communication between type II hair cells could modulate vestibular signaling. In addition, the Stone lab found that type II-lie hair cells with processes are the only hair cell type that is regenerated spontaneously in adult mouse vestibular epithelia after damage (Golub et al., 2012). In order to develop treatments for individuals with balance disorders, it is critical to define both the identity of spontaneously regenerated hair cells and the role that they play in vestibular processing. For this project, the Stone lab at UW and the Eatock lab at Harvard University propose to characterize the hair cell processes, to identify the hair cell type that bears them, and to begin to examine how these processes affect the coding of head movements and the transmission of sensory information from the inner ear to the brain. For both normal and regenerated states, the Stone lab will analyze morphological and molecular properties of hair cells with processes, while the Eatock lab will examine their physiological properties and define the types of connections that the processes make with nerves and other hair cells. Proposed studies in normal vestibular organs are essential steps in defining the structure of the novel processes and their functions in vestibular processing. Confirmation of hair cell-hair cell communication would transform our understanding of hair cell biology by allowing the possibility of lateral interactions as documented in the retina. Studies of damaged vestibular organs will help uncover the relationship between normal and regenerated hair cells with processes and the potential of regenerated hair cells to restore function in balance and hearing disorders.